1. Field of the Invention
The present invention relates to a process for producing ethylene-vinyl acetate copolymer continuously in a very stable manner over a long period of time.
2. Description of the Prior Art
Much has been studied so far of the technology that permits stable production of ethylene-vinyl acetate copolymer over a long period of time; but no satisfactory method has, as yet, been found. The key to the stable continuous operation is how to remove a large amount of reaction heat generated during the copolymerization of ethylene and vinyl acetate. There are several means for removing the reaction heat. In one method, reaction heat is removed by means of the jacket and coil provided on and in the polymerization vessel. In another method, reaction heat is removed by circulating the reaction liquid through a cooler installed outside the vessel. In a further method, reaction heat is removed by introducing ethylene gas, which is present in the upper space of the vessel, into a cooler installed outside the vessel, and returning the cooled or condensed ethylene to the vessel. These methods, however, have the following drawbacks:
(1) Drawbacks involved in the method of removing reaction heat by means of the jacket and coil attached to the polymerization vessel.
(a) With the lapse of operation time, polymer deposits on the surfaces of the jacket and coil, reducing their capability of heat conduction. To continue operation, the polymer deposit has to be washed out from the jacket and coil. This requires periodic shut-down.
(b) The flow of the reaction liquid is inevitably poor in the space between the pipes of the coil and the space between the coil and the vessel wall. This leads to the formation of gel and lumps that adversely affects the quality of the resulting polymer.
(2) Drawbacks involved in the method of removing reaction heat by means of the circulating cooler for the reaction liquid.
(a) As with the above case, polymer deposits on the heat transfer area of the circulating cooler with the lapse of operation time, reducing the capability of heat conduction. To continue operation, it is necessary to remove the polymer deposit or to install the cooler in duplicate for alternate use. The former requires periodic shut-down.
(b) Since the reaction solution is highly viscous, circulation thereof causes a great deal of pressure loss. Cooling by the evolution of sensible heat requires the circulation of a large amount of reaction solution. This leads to the consumption of a large amount of pumping energy. Pumping under high pressure often causes leakage from the seal. The leaked solution releases ethylene gas and solidifies on the equipment, entailing a danger and creating a difficulty in equipment maintenance.
(c) The high viscosity of the reaction solution makes it difficult to increase the flow rate, and the low thermal conductivity of the reaction solution requires the cooler to have a large heat transfer area.
(d) No coolers achieve complete piston flow because of the structure, and the hold-up in the dead space is inevitable where gel and lumps detrimental to the quality are formed.
(3) Drawbacks involved in removing reaction heat by cooling and condensing ethylene gas.
(a) The condensing temperature of ethylene is low (particularly when the pressure is low) and the condensation of ethylene is only possible by the aid of a cooling medium at an extremely low temperature.
(b) Since ethylene gas to be cooled has only a small amount of sensible heat, it is necessary to circulate a large amount of ethylene gas by using a booster.
(c) The condensation of ethylene is impossible to perform under a pressure higher than that (50.5 atm) at the critical point.
(d) As the state of ethylene approaches the critical point, the difference in enthalpy and density becomes small between gaseous ethylene and liquefied ethylene. This makes the condensing operation difficult.